Monitoring devices especially video recording devices are today widely used for monitoring of different premises, e.g. houses, road strips, metro stations or production plants. By recording a monitoring video sequence, information pertaining to an event, being e.g. a crime, an accident or a production miss failure, may be recapitulated by play back of the monitoring video sequence. One type of a video recording device is a digital monitoring camera being arranged to capture digital video sequences and to send the digital video sequences over a network e.g. the internet. Such a digital monitoring camera is typically set up to operate according to the following. The camera's components, (comprising a lens and an image sensor), capture images, which can be described as light of different wavelengths, and transforms them into electrical signals. These signals are then converted from analog to digital format. Thereafter the digital format signals are being subjected to digital image processing before being sent over the network.
A trend of today is that the cameras are equipped with better and better image sensors having higher and higher image resolution. This results in that the amount of monitoring digital video sequence data to be sent over the network is increasing. Moreover, the amount of monitoring digital video sequence data to be stored is increasing. Hence, the bit rate of the monitoring digital video sequence data is increasing. In order to reduce the bit rate of the monitoring digital video sequence data the monitoring digital video sequence data is typically compressed using video coding techniques such as video coding standards of the ISO/MPEG or ITU-H.26X families.
In modern digital video coding systems, two main modes are used to compress a video frame of a sequence of video frames: the INTRA-mode and the INTER-mode. In the INTRA-mode, the luminance and chrominance channels are encoded by exploiting the spatial redundancy of the pixels in a given channel of a single frame via prediction, transform and entropy coding. The INTER-mode, exploiting the temporal redundancy between separate frames, relies on a motion-compensation prediction technique that predicts parts of a frame from one (or more) previously decoded frame(s) by encoding the motion in pixels from one frame to the other for selected blocks. In INTER mode the luminance and chrominance channels does not have any motion predictions. A motion detection algorithm is typically used to detect motion between two frames when encoding using the INTER mode.
In U.S. Pat. No. 8,208,555, a method for reducing the bit rate of video data even further as compared with only using encoding is disclosed. An image capturing module is used for capturing a scene of an area to be displayed and outputting image data. An image processor converts the image data according to a first and a second cutting areas and a first and a second shrink ratio values, and outputs a first and a second sub-images. The first cutting area includes the second cutting area, and the first shrink ratio value is greater than the second shrink ratio value. An image encoder encodes the first and the second sub-images according to first and second encoding information, and outputs a first and a second digital video sequences. A network transmission module is used for transmitting the first and the second digital video sequences to a receiver. At the receiver the two digital video sequences are decoded and reconstructed into the original image data.
The method according to U.S. Pat. No. 8,208,555 provides reduced bit rate of the image data both during transmission of the image data from the sending device to the receiver and during storing of the image data in the form of the two image streams. However, a drawback with this method of reducing the bit rate is that in order to play back the captured scene comparably large processing power as compared with only decoding a single encoded digital video sequence is needed to both to decode the two digital video sequences and to reconstruct the image data from the decoded two digital video sequences.
Hence, in light of the above there is a need for a method to reduce the bit rate of monitoring digital video sequence data without the need to use comparably large processing power for play back of stored video data.